One insightful way to marry "simplicity of design and complexity of data" is to learn from the art of comics. I have been a fan of Scott McCloud's Understanding Comics for about five years now, and I continually find it a source of insights. I first heard about it through a marketing exec at IBM, who told me that Anil Menon, a VP of marketing at IBM, and a one time marketing academic whose work I respect very much, recommended it to all his staff.
Garr Reynolds has a rich post on learning from McCloud here. Reynolds notes the apparent tension between simplicity and complexity, when he shows a "before and after" example of a slide he drew:
In some ways the revised slide on the right is more complex, but from the point of view of its Gestalt, it's more powerful, simple, and easy to grasp quickly.
As I mentioned in my earlier post, though, the ideal for presentation design is to combine both simplicity and complexity on the same slide, to achieve (and I'm now paraphrasing Tufte slightly) simplicity of design and complexity of detail. (The earlier post is here).